Domestic sewage and industrial wastewater often contain impurities which include materials such as sugars and other carbohydrates and proteins and other forms of nitrogen. Many of these impurities or pollutants are decomposable by microorganisms, and there are various types of systems to remove the impurities from wastewater by action of microorganisms. One type of wastewater treatment system is known as an orbital system, sometimes referred to as an oxidation ditch system.
Orbital wastewater treatment systems include an elongated tank having two sidewalls and at least one partition wall mounted vertically in the tank substantially parallel to the two sidewalls and spaced apart from the ends of the tank. The tank and partition wall together form an endless, circuitous channel to contain a stream of mixed liquor. An orbital wastewater treatment system also includes means to cause the liquid to flow through the channel and an aerating means to introduce air into the liquid to provide oxygen for the microorganisms. Various mechanisms can be used for such purposes including a surface aerator, a rotary perforated disc-type aeration mixer, or a rotating brush aerator. Orbital wastewater treatment systems are taught, for example, in U.S. Pat. Nos. 3,510,110 and 3,846,292. According to U.S. Pat. No. 3,846,292, influent wastewater is introduced into the tank and driven to flow around the endless, circuitous channel. A stream of treated liquid, less than the total flow in the channel, is removed from the channel and transferred into a solid-liquid separator, or clarifier, spaced apart from the orbital system. In the separator, solid particles form sludge which settles, and part of the sludge is returned to the orbital system to mix with the wastewater to form mixed liquor. Clarified liquid is transferred from the separator to a stream or other body of water or sent to further treatment. The purpose of returning sludge to the orbital system is to maintain a predetermined concentration of microorganisms in the mixed liquor, thereby to accomplish biological removal of pollutants from the wastewater.
The construction of a conventional orbital system such as taught in the above patents requires that the circuitous channel be constructed and that a separate sludge separation system also be added. Additionally, a system of pipes and pumps must be installed to permit diversion of wastewater from the orbital system to the separator and for return of sludge from the separator to the orbital system.
As discussed in U.S. Pat. No. 4,303,516 (Stensel, et al.) assigned to a predecessor of applicants' assignee and as practiced in 1981 at a wastewater treatment plant at Campbellsville, Kentucky, and later at Owensboro, Kentucky, a rectangularly shaped clarifier is disposed in an orbital channel between a channel wall and partition with a frontal top weir to receive a portion of the overall mixed liquor flowing through the orbital channel. The mixed liquor portion passed down the clarifier co-currently with the main orbital channel flow into a clarifier quiescent zone. Biological sludge is settled in the clarifier for removal through ports in the clarifier bottom back in to the orbital channel mixed liquor flow and clarified liquor removed by overflow into effluent troughs alongside the partition wall and extending over a substantial length of the top of the clarifier. In the '516 patent, one embodiment employs a dipped orbital channel portion so that the orbital channel flow has substantially the same cross-sectional area throughout its length. In the Campbellsville installation, the clarifier was installed over the same channel floor elevation as the remainder of the orbital channel, and thus the remaining orbital channel flow at that location had less of a cross-sectional area than the remaining areas of the channels.
U.S. Pat. Nos. 4,362,625; 4,383,922; 4,457,849; and 4,780,206 to Beard also show intra-channel clarifiers which involve a boat-shaped structure positioned in a channel with its bow directed into the wastewater flow and providing a stern or rear inlet for entry of a portion of the wastewater flow, with that portion being flowed in the clarifier counter-currently to the channel flow at the bow of the clarifier structure. Sludge is settled in the clarifier and flows back into the channel between rows of vertical plates or through tubes. An effluent launder is positioned in a forward bow section of the clarifier structure.
Various other types of intra-channel clarifiers, including modifications of the Stensel, et al., patent and the Beard boat structure, are shown and discussed in an article entitled "Assessment of Design Trade-offs When Using Intrachannel Clarifiers" by Jon H. Bender of the U.S. Environmental Protection Agency, published in the October 1987 issue of the Journal WPCF volume 59, number 10, pages 871-876. As set forth in the article, an intra-channel clarifier must not negatively impact the mixed liquor flow velocity in the orbital channel. All intra-channel clarifiers restrict the circulating flow of mixed liquor in the aeration channel to a certain degree. Such restrictions must be minimized to maintain adequate channel velocities without requiring additional power to the aerator or other propelling means for the mixed liquor. The capability of the aeration device to overcome headlosses in the channel also must be considered.
The article also indicates that consideration must also be given to the effect of aerator channel and clarifier maintenance in an intra-channel clarifier system. Proper adjustment of sludge return flows from the clarifier to the orbital channel are also a factor. Paramount to any intrachannel clarifier is the cost-effectiveness in terms of original cost, operational (power) costs, operational manpower costs, maintenance cost, and longevity.